Picture the scene: a young Scots couple go camping in the woods. Campbell Armstrong, a precocious teenager, has invited his first girlfriend to spend the night with him in a tent. Eileen (23) - Jewish, five years his senior - reveals a scar on her stomach: the result of a Caesarean section. She was pregnant at 16 and her mother forced her to give the baby up for adoption. Little does Campbell know that years later, long after the breakdown of his marriage to Eileen, he will write a book about Eileen's lost baby, Barbara.

All That Really Matters is Armstrong's account of the extraordinary story of Barbara's discovery of her mother at the very moment when both of them were seriously ill with cancer. Barbara, 42 years old and with three children of her own, was living in Yorkshire. Eileen, divorced from Armstrong (with whom she'd had three sons), was living in Phoenix, Arizona, 6,000 miles away. The book begins in the summer of 1997 with the revelation that Eileen has lung cancer. Campbell is living in a Victorian house near Birr, Co Offaly, with his second wife, Rebecca. A Glaswegian who retains a soft Scots accent, he spent 20 years in the US before moving to Ireland in 1991. His is a novelist who normally writes "dark novels of suspense". He wrote All That Really Matters on Eileen's request: "She was going to write the book herself, about her reunion with Barbara, but she was getting too weak. She had only written a couple of pages about the adoption, and I took it up from there. I was quite shocked. It was not the kind of book I thought I'd ever write."

The book is deeply personal, as much about Campbell's making peace with his own hectic, drinking past as it is about Eileen's rediscovery of her lost firstborn: "I did try to leave myself out of it," he admits, "but it started to sound like a pulp novel, you know, with the young heroine rushed away to a distant seaside town to have her illegitimate baby. So I wrote myself back in, and that meant I had to explain a lot about myself too." Every turn of this baffling story bears out the old adage that truth is stranger than fiction.

Eileen and he, although divorced, remained friendly, and he was shocked to hear of her illness: "How little I know about cancer: it's what happens in the next house, the next street. It's what strikes down distant aunts, cousins you barely know. When it comes to perch on your own roof, you have to accommodate its strange force because you can't demand that it leave. It's the deadly gate-crasher that stays until the party is well and truly done." The poignant story of Eileen's bravery and spirit as she copes with her illness dovetails with Barbara's.

Being diagnosed with ovarian cancer made Barbara all the more determined to find her mother. After years of fruitless searches in England, Wales and Northern Ireland, she managed to track down Sydney, Eileen's brother, in Glasgow. Sydney, afraid that news of Barbara might upset his ill sister, kept Barbara's letter for two agonising months before sending it on to her mother.

But then, one night, Barbara received the call she'd been waiting for all her life: "Is this Barbara? This is your mother, Eileen." Barbara wrote in her journal: "This is the most powerful feeling I have ever experienced. The words `This is your mother' - these were more intense than the births of my children. One of the first things my mother said was that she hadn't realised how deeply she'd buried the pain and locked it away for 42 years. And she hadn't looked for me because she feared I'd reject her because I'd blame her for giving me up. I told her I never blamed her once."

On hearing about Barbara, Eileen had cried with joy and relief. "She had prayed for news of her daughter, and her prayers had been answered, just like that, just when you least expected an answer, out there in the vast silence of the universe." Barbara's resources were limited, but she managed to scrape together the money to see her mother three times before Eileen died. Barbara was self-conscious about her appearance: she wanted to look her best, meeting her mother for the first time, but she was bloated and overweight due to heavy doses of steroids and HRT; and chemotherapy had left her with nothing but fluff on her head.

Eileen said: "I remember this head, you know. The last time I saw you, your hair was just like this." Although dissimilar physically, the two women felt an instant rapport, spending hours on "girlie" activities like shopping and hairdressing; talking incessantly. Eileen was able to tell her daughter that Barbara's father was a man whom Eileen had loved, but who had abandoned her on learning of the pregnancy. Barbara had thought her mother was a victim of rape. This was the story concocted by Eileen's mother - determined to the last to seem respectable - to explain to the adoption agency why Eileen was giving up her child.

Mother and daughter had four short months to catch up on the events of more than 40 years. Barbara battled on with her own cancer after having nursed her dying mother. She died last September, a year-and-a-half after Eileen: "Barbara held on for a long time: she was very strong and determined," says Armstrong. "She read my book and liked it. I included a lot of what she had written herself about the experience, and she was proud to see her own words in print. She was also pleased to see the cover, which is a photograph of herself as a child. She had it on the bedside table in the hospital before she died." Of Barbara and Eileen's brief, intense reunion, Armstrong concludes: "It was as if a bizarre clock ticked at the heart of the universe, one whose mechanism worked in synchronicity very rarely, perhaps only once in anyone's lifetime, and that split second had happened in the lives of Eileen and Barbara, that fragment of time when the hands of the clock come together."

All that Really Matters by Campbell Armstrong will be published by Little Brown, price £14.99 in UK, on January 20th